1. Field of the Invention
The present invention relates to a nonaqueous battery. More particularly, it relates to a nonaqueous battery including a negative electrode containing a carbon material capable of absorbing/desorbing lithium, metallic lithium or an lithium alloy, a positive electrode containing a chalcogenide, and a nonaqueous ionic conductor.
2. Description of Related Arts
In recent years, with increasing miniaturization of portable electronic devices, there is demand for, as power sources therefor, batteries of high voltage and high energy density which are capable of operating within a wide temperature range in consideration of environments in which they are used. For example, lithium batteries using metallic lithium as a negative-electrode active material have been developed and are now in use for cameras and memory back-up.
Lithium-ion secondary batteries capable of charging/discharging by use of a carbon material allowing lithium to be absorbed/desorbed or inserted/released have also been put into use recently. Nonaqueous ionic conductors used in these batteries are roughly classified into three types, that is, an electrolyte solution type containing a lithium salt dissolved in an organic solvent, a solid electrolyte type and a molten salt type. Generally the electrolyte solution type and the solid electrolyte are used.
However, further improvement in the quality of lithium secondary batteries is still demanded. For this purpose, the nonaqueous ionic conductor is expected to possess the following characteristics:
(1) Being stable to a positive and a negative electrode PA1 (2) Exhibiting a high ionic conductivity in a broad temperature range PA1 (3) Having a high boiling temperature or a low vapor pressure PA1 (4) Being highly safe.
In nonaqueous ionic conductors of the electrolyte solution type, a non-protonic solvent which is stable with respect to the negative electrode and a solvent of a chain ether or of a carbonate which is stable with respect to the positive electrode are used for ensuring the characteristic (1). For ensuring the characteristic (2), a solvent mixture containing a solvent having a high dielectric constant and a solvent having a low viscosity is used. For ensuring the characteristic (3), substituent groups are introduced to a known solvent or the mixture ratio of the solvent having a low viscosity is optimized. For ensuring the characteristic (4), a chemically stable substance is used as an electrolytic salt.
As for nonaqueous ionic conductors of the solid electrolyte type, which has the characteristics (3) or (4) by nature, it has been proposed that polyethylene oxide, polypropylene oxide or the like be used in a basic composition for ensuring the characteristic (1) and that an organic solvent be added to the basis composition to improve the ionic conductivity for ensuring the characteristic (2).
From the above-mentioned viewpoints, it has been proposed for nonaqueous ionic conductors of the electrolyte solution type that a mixture solvent be used which contains a non-protonic solvent having a high dielectric constant and capable of dissolving an electrolytic salt as well as a chain ether or a chain ester for decreasing the viscosity of the non-protonic solvent and thereby improving the ionic conductivity at low temperatures.
For example, Japanese Unexamined Patent Publication No. HEI3(1991)-59962 discloses use of an ethylene glycol alkyl ether or a diethylene glycol alkyl ether in a lithium secondary battery whose positive-electrode active material is an electrically conductive polymer, for improving the ionic conductivity.
However, there remains a problem in that the electrically conductive polymer composing the positive electrode is low in density and the secondary battery using such a positive electrode is lower in energy density than the one using a positive electrode containing a chalcogenide.
Japanese Unexamined Patent Publication No. HEI 4(1992)-162363 discloses a combination use of 1,2-dimethoxypropane with a carbonate for improving the ionic conductivity in a broader temperature range.
However, 1,2-dimethoxypropane is highly reactive to a graphite type negative electrode. Therefore, a sufficient performance is not obtained.
Further, Japanese Unexamined Patent Publication No. HEI 4(1992)-206270 discloses a combination use of a dialkoxyethane with a nonaqueous electrolyte solution for suppressing the reactivity of the electrolyte solution with the positive or negative electrode. Japanese Unexamined Patent Publication No. HEI 5(1993)-82167 proposes a combination use of propylene carbonate with dimethyl carbonate for suppressing the reactivity with the positive electrode. Japanese Unexamined Patent Publication No. HEI 4(1992)-280082 proposes use of dipropyl carbonate in a mixture solvent for suppressing the reactivity with metallic lithium or a carbon material having absorbed lithium which carbon material is hard to graphitize.
As regards ionic conductors of the solid electrolyte type, Japanese Unexamined Patent Publication Nos. SHO 61(1986)-214374 and SHO 62(1987)-278774 propose the complexing of a nonaqueous solvent with a polyalkyl methacrylate which is a polymeric solid electrolyte and the complexing of propylene carbonate or acetonitrile with polyacrylonitrile or polyvinylidene fluoride, respectively, for improving the ionic conductivity at low temperatures.
As described above, a variety of nonaqueous ionic conductors of the electrolyte solution type and of the solid electrolyte type have been proposed. However, no ionic conductors have not been realized yet that satisfies the requirements of being stable both to the negative-electrode active materials such as metallic lithium, a lithium alloy or a carbon material in which lithium is absorbed or inserted and to the positive-electrode active material such as a chalcogenide, exhibiting a high ionic conductivity in a broad temperature range and having an excellent stability at high temperatures.